The "lumped mass" versus "consistent mass" option controls how the "mass matrix" is formulated. (Read the "help text" on this item for details.) I'd use the "consistent mass" option - I believe it is more accurate.

I really don't like using the "lumped mass" input dialog. Typically anytime you see a value here, it represents something that should have been modeled differently upfront in the (static) piping model. CAESAR II will automatically account for the mass of the piping system and components that have been modeled. So the mass of the rigid elements will be accounted for - you don't have to do this in the Dynamic Input.

What we often see is that after building the static model, someone discovers a relatively heavy valve actuator exists. They put this into the static model as a concentrated force. After moving on to Dynamics, they realize forces are not masses, and use the Lumped Mass dialog to again define actuator. A better solution would have been to use dummy rigids to position the actuator, then another dummy rigid for the actuator in the static model. Then the mass of the actuator would have been automatically accounted for in Dynamics.